The present invention relates to liquid crystal display apparatuses, and more particularly, to liquid crystal display apparatuses having light collecting mechanisms.
Liquid crystal display (LCD) apparatuses, which display images by controlling voltage, are made compact and thin and do not consume much electric power. Therefore, LCD apparatuses are often applied to office automation (OA) and audio visual (AV) equipment.
FIG. 1 is a schematic cross-sectional view showing a prior art LCD panel 100. The LCD panel 100 includes a first glass substrate 101 on which a plurality of pixel electrodes 102 are arranged. An orientation film 103 is applied to the first glass substrate 101 to cover the pixel electrodes 102. A second glass substrate 104, on which a common electrode 105 is formed, faces the first glass substrate 101. An orientation film 106 is applied to the common electrode 105. Liquid crystal 107 is sealed between the orientation films 103, 106.
The liquid crystal 107 has electro-optic anisotropy. Therefore, if voltage is applied to the pixel electrodes 102 and the common electrode 105, an electric field is applied to the liquid crystal 107 by the pixel electrodes 102 and the common electrode 105. The intensity of the electric field determines the transmittance of light. The LCD panel 100 is illuminated from behind by a luminescent portion, or backlight, to form a display image with each pixel having a predetermined brightness.
A liquid crystal display apparatus may be applied to the monitor of a portable television or a video camera, which are frequently used outdoors. Therefore, ambient light may be used to form display images on such monitors.
FIG. 2 is a cross-sectional view showing a prior art LCD apparatus that incorporates a light collecting mechanism. The LCD apparatus has an LCD panel 100 and a backlight 110, which are attached to a case 120. The backlight 110 includes a light guide plate 111 arranged adjacent to the LCD panel 100, a light source 114 located at a first end of the light guide plate 111, and a light collector 115 optically connected to a second end of the light guide plate 111. The light guide plate 111 includes a diffusing portion 112 facing the LCD panel 100 and a reflecting portion 113, which is arranged on the opposite side of the light guide plate 111. The display area of the LCD panel 100 and the light collector 115 are exposed from the case 120. The LCD panel 100 also includes a control circuit (not shown), which receives image signals and applies voltage to each pixel electrode.
It is preferred that a light emitting diode (LED), an electroluminescent (EL) device, or a fluorescent lamp be employed as the light source 114. A reflector 116 is arranged behind the light source 114. It is preferred that the light guide plate 111 be made of a transparent material such as acrylic resin, polycarbonate, or glass. The diffusing portion 112 and the reflecting portion 113 need not be formed separately from the light guide plate 111 and may be formed integrally with the light guide plate 111 by carrying out a diffusing treatment and an irregular reflection treatment. The light collector 115 draws ambient light, such as sunlight or indoor light, toward the light guide plate 111. Furthermore, the light collector 115 includes a lens that may be formed either integrally with or separately from the light guide plate 111.
The internal light from the light source 114 and the ambient light from the light collector 115 are transmitted to the light guide plate 111 and irregularly reflected by the reflecting portion 113. Some of the irregularly reflected light is diffused by the diffusing portion 112 and irradiated toward the LCD panel 100. The rest of the irregularly reflected light is reflected toward the reflection portion 113. In this manner, the internal or ambient light drawn toward the light guide plate 111 attenuates as it proceeds back and forth horizontally parallel to the plane of the drawing between the diffusing portion 112 and the reflecting portion 113. Images are displayed by controlling the transmittance of the light irradiated toward the LCD panel 100 from behind.
Ambient light is abundant on sunny days. Therefore, if the LCD apparatus is used outdoors on a sunny day, the light source 114 is not used. In other words, the LCD panel 100 is illuminated solely by the light collected through the light collector 115. The LCD panel 100 may be illuminated by the light source 114 when using the LCD apparatus indoors, where the amount of light is insufficient. Accordingly, the usage of ambient light decreases the total electric power consumption since the light source 114 need not be used so frequently.
However, turning the light source 114 on and off manually is troublesome. Furthermore, if the LCD apparatus is seldom used outdoors, the light source 114 is lit for a longer period of time. Thus, power consumption cannot be decreased effectively. Additionally, selective usage between the light source 114 and ambient light makes it difficult to maintain the image quality.
The image quality of the LCD panel 100 depends greatly on the contrast ratio and the brightness of image. The contrast ratio is the ratio of the maximum transmittance to the minimum transmittance. The image becomes clear when the contrast ratio is high. The brightness of image refers to the brightness of the display that is determined by the maximum transmittance and the illuminance of the backlight. When using ambient light, the illuminance of the backlight changes in accordance with the brightness of the ambient light. However, the voltage applied to the pixel electrodes 102 and the common electrode 105 remains the same even if the amount of ambient light changes. That is, the transmittance remains the same even if the amount of ambient light changes. This results in a nonuniform image quality.
Furthermore, the illuminance of the light source 114 is lower than that of the ambient light. Therefore, if the contrast ratio and brightness are determined using a criterion set for the ambient light, the illuminance of the light source is increased excessively. This increases the power consumption of the LCD panel 100. On the other hand, if the contrast ratio and brightness are determined using a criterion set for the light source 114, the passage of ambient light may not be blocked even if the transmittance is set to a minimum to display a black color. Further, the intensity of the transmission light may become too high if the transmittance is set to a maximum to display a white color. This results in a decreased visibility.